Method for preparing oxidized cellulose



Patented Sept. 16, 1 941 umrsn STATE mz'rnon FOB ranrsnme. oxmrzsn csnnmosa Gordon D. Biatt, Rochester, N. Y assignor to Eastman Kodak Company, Rochester, N. Y., a corporation ofNew Jersey No Drawing.

.5 Claims;

this means alcohol groups are oxidized to carboxyl groups so that the resulting productis a polycarboxyl compound.

An object of the present invention is to pro vide a method of preparing an oxidized cellulose of this nature in which the N01 is employed inliquid form. Another object of my invention is to provide a process for preparing an oxidized cellulose in which the crude liquid N: from the recovery of nitric acid in a nitrating plant can .be utilized in the process. A further object of' my invention is to provide a simple and practical method for preparing oxidized cellulose which is soluble in dilute alkali and has a high CO: equivalence, which method requires less attention and less complicated apparatus than has formerly been employed in a process of this nature.

The prior art would seem to indicate that the treatment of cellulose with liquid NO: alone, instead of oxidizing it, nitrates the cellulose. For instance, in U. S, Patent No.1,917,400 cellulose is .treated with liquid N02 for a time and a .considerable amount of nitration occurs; I have found, however, that when cellulose is treated with a mixture of liquid N0: and 545% of nitric acid, it is oxidized to a polycarboxyl compound a and substantially no nitration occurs.

stantially no nitration is meant that the amount of nitrogen, if any, added to the cellulose is so small that analysis fails to show 'a nitrogen cons tent of as much as one percent. This oxidation is carried out with a liquid free of materials which would promote nitration. For instance, neither acetic anhydride nor sulfuric acid should be present. I have found that by allowing cellulose to remain in a liquid essentially consisting of liquid N0: and a small proportion of HNO: for

(aqueous sodium hydroxide of 2% concentration) is obtained. It is necessary that the so-treated cellulose have a carboxyl content of 11.5% or more to obtain complete solubility. with carboxyl contents appreciably less than that, only By sub-.

Application December 5, 1940, Serial No. 368,662

cellulose, having a carboxyl content near to but not quite 11.5%, the product will dissolve, except for possibly a few fibers. If, however, the carboxyl content is considerably below 11.5%, a con-' siderable proportion of undissolved material will be present. As the oxidized cellulose prepared by my invention is ordinarily useful,because of its alkali solubility, sufilcient oxidation to" give a carboxyl content of at least 11.5% is desirable. Ordinarily, approximately? hours is required to obtain an oxidized cellulose having a carboxyl content of 11.5%. If an oxidized cellulose is desired, having a carboxyl content of approximately 13%, the treatment with the mixture of liquid N0: and nitric acid should ordinarily be nearly 10 hours. I To obtain a product having a carboxyl content of approximately 15%, a time of treatment of about 12 hours is desirable. For a prodnot having a carboxyl content of approximately 18-19%, the treatment should be for approximately 20-25 hours. A longer time of treatment will result in further increase of carboxyl. These figures are merely representative and may vary somewhat with varying conditions. When all of the primary alcohol groups of a cellulose are oxidized to carboxyl groups, the material analyzes 25.6% carboxyl. In actual practice, however, this theoretical amount is approached but has not been reached. The products in which the highest degree of oxidation has been obtained contain 2023% carboxyl. Since ordinarily, alkali solubility is all that is desired, continued oxidation,

after good alkali solubility has been reached.

serves 'no useful purpose. In this application where terms such as "NOr and nitrogen tetroxide are employed, .they refer to either NO: or its parts of this mixture toevery' part of cellulose a time, that a product soluble in dilute'alkali than four parts of liquid would not "be objecto, insure proper wetting of the cellulose. Any amount of this mixture above four parts per part of cellulose may be employed. However, the use 'of more than six parts is wasteful and, therefore, 4-6 parts is the preferred range. If some typeof mixer is employed which insures intimate mixing of theliquid and the cellulose. the use of less tionable, the only criterion being thatsuiiicient partial solubility is obtained. with an oxidized After the cellulose and liquid are intimately mixed, the mix is allowed to stand for the desired time at any suitable temperature. Any temperature between the solidifying point'of the 'mass and the boiling point of the N02 may be point of the N02 due to the evolution of'gas which must be taken care of. In carrying out the process, it is ordinarily desirable either to use a closed vessel, which will withstand any pressure which might be generated in the reaction, or to equip the vessel with a good refluxing apparatus so that theNOz does not escape. If, however, economy is no object, the process might be carried out in a container having a small opening which is open to the air, particularly where the temperature is kept low so that the N02 does not escape in appreciable quantities. The reaction may be carried out in the complete absence of water, or a small amount of moisture may be present. It is preferable, however, to keep the moisture low, for example not more than 3% of moisture. The crude NOz, which is ordinarily employed in'my process; is inherently anhydrous and, therefore, any moisture, which may be present as my process is ordinarily carried out, would be that supplied by the cellulose.

As pointed out before, the liquid, which I have found quite suitable for use and which I prefer to employ, is the crude N02 from the acid recoverypart of a nitrating plant. When the spent acid in such a plant is distilled to. separate the nitric acid from the sulfuric acid or other foreign materials, the distillate, which comes over, essentially consists of N02 and nitric acid. The

amount of nitric acid which is compatible with liquid N02 is limited and therefore the distilled crude recovered acid forms into two layers, the upper layer consisting of N02 containing the maximum amount of nitric acid which forms'a homogeneous mixture therewith. At 11 C. the percentage of nitricacid which will be present in this layer is between 6%-and 7%, while at 18 C. the percentage is approximately 8%. This upper layer may be separated off and used for oxidizing cellulose in accordance with my invention.

It is often desirable to employ a greater percentage .of nitric acid than will form a homoge-- neous solution of HNOs in the liquid N02. For instance, a percentage of HNOa up to 15% has been found suitable for use in theoxidizing liquid employed to oxidize cellulose in accordance with my invention. In that case it is desirable to thoroughlymix, at least the excess, HNO3 with the oxidizing liquid shortly before putting it on the cellulose so that an intimate mixture of the HNO: and liquid N02 is obtained.

If the upper=-layer from the distilled recovered nitrating acid from the acid recovery part of a nitrating plant is used and a, greater proportion of nitric acid, than is there present, is desired in the oxidizing liquid, either a sinall amount of the liquid forming the lower layer of the distilled acid or nitric acid per se may be added. I have found, however, that at least of nitric acid should be present in the N02 with which the cellulose is treated to inhibit any appreciable degree of ultra tion of the cellulose After the cellulose has been treated the desired which depends primarily on the time of treat should be in a physical condition that allows of v at 14-24 C. for 25 hours.

time, it may be aerated so that the N02 may be driven off. Any N02 and nitric acid-remaining may then be removed by thoroughly washing the oxidized cellulose with water. The resulting product has a carboxyl content the degree of easy penetration of the liquid thereto, such as a. flufied condition. I have employed cotton linters, cotton and wood pulp in fiuffed condition and have found all of these various types of cellulose to be suitable for use in my process. lose is so compact that the oxidizing liquid has difiiculty in penetrating therein, the oxidation is non-uniform and ordinarily uniformity is desirable. However, if non-uniformity is not objectionable, cellulose in compact form may also be oxidized by my process.

The following examples illustrate my invention: Example I Five grams of cotton linters, containing 2-3% of moisture, was added to 25 cc. of. liquid N02 containing 5% of nitric acid. This mixture was maintained at 23 C. for 7 hours with consequent loss of oxides of nitrogen. The product was aerated and washed with distilled water to remove all trace of the reaction mixture. This oxidized cotton which was completely soluble in aqueous sodium hydroxide solution 013 2% concentration contained 13.4% carboxyl.

Example II 2% concentration and was found to contain 16.5% of carboxyl.

Example III Three pounds of oven-dried cotton linters was thoroughly mixed with 13 pounds of a mixture consisting of of N204 and-10% of HNO: to thoroughly wet the fibers. This mixture was pressed into-a closed crock and allowed to stand The washed and purified material from this reaction analyzed 20% carboxyl. .ihe fibers were completely soluble in 2% alkali.

The oxidized cellulose prepared in accordance with my invention may be employed in any connection for which a material of this type is suitable, as disclosed inthe Yackel and Kenyon application Serial No. 219,464. These oxidized celluloses, having a carboxyl content, may be combined withbases, either organic or inorganic, to form salts or esters, as desired.

I claim:

1. A method of preparing an. oxidized cellulose in which alcohol. groups of the cellulose are oxidized to-carboxyl groups which comprises sub- -jecting the cellulose to alluding treatment and subsequently treating it for at least 7 hours with If the cellulose tostand mixed with at least 4 parts of liquid NO: containing 5-15% of nitric acid until th cellulose is completely alkali soluble. l

5. A method of preparing an oxidized cellulose in which alcohol groups of the cellulose are oxidized to carboxyl groups by treating the cellulose with at least 4 parts of a mixture essentially 5 consisting of liquid N02 and 10% of nitric acid.

GORDON D. HIATT. 

